docs: def assert_boolean_false(expr, msg_fmt="{msg}"):
      """Fail the test unless the expression is the constant False.
  
      >>> assert_boolean_false(False)
      >>> assert_boolean_false(0)
      Traceback (most recent call last):
          ...
      AssertionError: 0 is not False
  
      The following msg_fmt arguments are supported:
      * msg - the default error message
      * expr - tested expression
      """
  
      if expr is not False:
          msg = "{!r} is not False".format(expr)
          fail(msg_fmt.format(msg=msg, expr=expr))